Apparatus for reducing emissions and method of assembly

ABSTRACT

A heat recovery steam generator (HRSG) is coupled to a gas turbine engine that is configured to combust a fuel in air to produce shaft power and a flow of exhaust gases including oxides of nitrogen (NO x ). The HRSG includes at least one duct burner for heating the exhaust gases and at least one NOx reduction element coupled downstream from the at least one duct burner and configured to facilitate reducing an amount of NOx in the exhaust gases that are channeled into the at least one NOx reduction element.

BACKGROUND OF THE INVENTION

The embodiments described herein relate generally to emissions treatment systems and, more particularly, to an apparatus for use in reducing NO₂ formation in the exhaust path of a combustion system.

During the combustion of natural gas and liquid fuels, pollutants such as, but not limited to, carbon monoxide (CO), unburned hydrocarbons (UHC), and oxides of nitrogen (NO_(x)) emissions, may be formed and/or emitted into an ambient atmosphere. In general, CO and UHC may be formed during combustion conditions at lower temperatures and/or during combustion conditions when an insufficient amount of time to complete a reaction is available. In contrast, NO_(x) is generally formed during combustion conditions at higher temperatures. At least some known pollutant emission sources include industrial boilers and furnaces, reciprocating engines, gas turbine engines, and/or steam generators.

Modern air quality regulations increasingly mandate reduced emission levels for power generation plants, while also requiring increased fuel efficiency requirements. To comply with stringent emission control standards, it is desirable to control NO_(x) emissions by suppressing the formation of NO_(x) emissions. Oxides of nitrogen include nitric oxide (NO) and nitrogen dioxide (NO₂), which is known to produce a visible yellow plume from exhaust stacks and that is alleged to contribute to the creation of “acid rain.” However, known combustion controls may provide only limited emissions control and may prove inadequate in satisfying the increased standards and the often-conflicting goals, such that further improvements of post-combustion exhaust gas treatment systems are desirable.

One known technology for use in controlling NO_(x) in stack emissions is selective catalytic reduction (SCR). In an SCR system, flue gases from power generation plants often have a net oxidizing effect due to a high proportion of oxygen that is provided to ensure adequate combustion of a hydrocarbon fuel. Thus, NO_(x) that is present in the flue gas may be reduced to nitrogen and water with great difficulty. An SCR element may be used to mix anhydrous ammonia with the flue gas, and the gases are channeled over a suitable reduction catalyst at a suitable temperature prior to being released into the atmosphere. However, the reaction rate over the catalyst is dependent on the inlet gas temperature as such the rate of NO_(x) destruction is insufficient until the flue gas is heated to the suitable temperature. Accordingly, during transient phases, such as during startup operations, the SCR element generally does not reduce the NO_(x) to a desired level as the flue gas temperature is to low.

BRIEF DESCRIPTION OF THE INVENTION

In one aspect, a method is provided for providing a heat recovery steam generator (HRSG) for use with a gas turbine engine. The method includes coupling at least one duct burner within the HRSG and coupling at least one oxides of nitrogen (NO_(x)) reduction element downstream from the at least one duct burner, wherein the at least one duct burner is configured to operate gas turbine engine operations including startup, shutdown, and low-load operations with insufficient exhaust temperatures at the NO_(x) catalyst inlet for desired NO_(x) reduction to increase a temperature of exhaust gases routed through the at least one NO_(x) reduction element to enable the at least one NO_(x) reduction element to facilitate NO_(x) reduction reactions during the gas turbine engine operations.

In another aspect, a heat recovery steam generator (HRSG) is provided, wherein the HRSG is coupled to a gas turbine engine that discharges a flow of exhaust gases including oxides of nitrogen (NO_(x)). The HRSG includes at least one duct burner for heating the exhaust gases and at least one NO_(x) reduction element coupled downstream from the at least one duct burner and configured to facilitate reducing an amount of NO_(x) in the exhaust gases that are channeled into the at least one NO_(x) reduction element.

In another aspect, a combined cycle power plant is provided, including a gas turbine engine and a heat recovery steam generator (HRSG) coupled in flow communication with the gas turbine engine. The gas turbine engine is configured to combust a fuel in air to produce shaft power and a flow of exhaust gases including oxides of nitrogen (NO_(x)). The HRSG includes at least one duct burner for heating the exhaust gases and at least one NO_(x) reduction element coupled downstream from the at least one duct burner and configured to facilitate reducing an amount of NO_(x) in the exhaust gases channeled into the at least one NO_(x) reduction element.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a simplified schematic diagram of an exemplary combined cycle power plant;

FIG. 2 is a block schematic diagram of an exemplary heat recovery steam generator (HRSG) that may be used with the combined cycle power plant shown in FIG. 1;

FIG. 3 is a simplified block diagram and an emissions profile for the HRSG shown in FIG. 2;

FIG. 4 is a side view of an exemplary second duct burner that may be used with the HRSG shown in FIGS. 2 and 3;

FIG. 5 is a graph showing emissions curves of NO_(x) emissions over time from the combined cycle power plant shown in FIG. 1; and

FIG. 6 is a graph showing an amount of NO_(x) destruction based on an SCR inlet temperature.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 a simplified schematic diagram of an exemplary combined cycle power plant 100. In the exemplary embodiment, power plant 100 includes a compressor 102 including an air intake 104 that receives air. Compressor 102 is coupled to a gas turbine engine 106 that includes one or more combustion chambers 108. Compressor 102 compresses air received via air intake 104 and channels the compressed air into combustion chambers 108, wherein the compressed air is mixed with fuel and ignited to supply gas turbine engine 106 with hot combustion gases for driving a first shaft 110. First shaft 110 is coupled to a first generator 112, and causes first generator 112 to generate electricity. Moreover, gas turbine engine 106 discharges exhaust gases into an exhaust duct 114, including, for example and not by way of limitation, oxides of nitrogen (NO_(x)), carbon monoxide (CO), and unburned hydrocarbons.

In the exemplary embodiment, power plant 100 also includes a heat recovery steam generator (HRSG) 116 that is coupled in flow communication to gas turbine engine 106. Specifically, HRSG 116 is coupled to gas turbine engine 106 via exhaust duct 114 such that HRSG 116 receives the exhaust gases discharged from gas turbine engine 106. In the exemplary embodiment, HRSG 116 includes one or more heat exchangers 118 and emissions treatment equipment 120. Heat exchangers 118 extract heat from the exhaust gases, and the heat is used to generate steam. Emissions treatment equipment 120 processes the exhaust gases, and the processed exhaust gases are subsequently released to the atmosphere via an exhaust stack 122.

A steam turbine 124 is coupled to HRSG 116 such that steam generated by heat exchangers 118 is channeled into steam turbine 124 for use in driving rotation of a second shaft 126. Second shaft 126 is also coupled to a second generator 128, and causes second generator 128 to generate electricity. The spent steam is then channeled into a condenser 130 that includes a plurality of tube bundles 132. Cooling water channeled through tube bundles 132 cools the steam such that the steam condenses into water. The water is then channeled back to heat exchangers 118.

FIG. 2 is a block schematic diagram of HRSG 116. In the exemplary embodiment, HRSG 116 receives a flow of exhaust gases discharged from gas turbine engine 106 (shown in FIG. 1) via exhaust duct 114 (shown in FIG. 1). Moreover, in the exemplary embodiment, heat exchangers 118 include a plurality of superheater heat exchangers 202, a plurality of reheater heat exchangers 204, and a plurality of economizer heat exchangers 206. HRSG 116 also includes a high pressure evaporator 208, an intermediate pressure evaporator 210, and a low pressure evaporator 212 that each produce steam using the heat contained in the exhaust gases. Each evaporator 208, 210, and 212 is coupled to a respective pressure drum. In the exemplary embodiment, high pressure evaporator 208 is coupled to a high pressure drum 214, intermediate pressure evaporator 210 is coupled to an intermediate pressure drum 216, and low pressure evaporator 212 is coupled to a low pressure drum 218. HRSG 116 also includes at least one duct burner 220 that supplies heat into the exhaust gas flow to enhance steam production output. Accordingly, in the exemplary embodiment, HRSG 116 generates steam at a plurality of different pressures using high pressure drum 214, intermediate pressure drum 216, and low pressure drum 218. Moreover, in the exemplary embodiment, each pressure drum 214, 216, and 218 routes the pressurized steam to a different steam turbine (not shown). In an alternative embodiment, each pressure drum 214, 216, and 218 channels the pressurized steam to a single steam turbine, such as steam turbine 124 (shown in FIG. 1). In the exemplary embodiment, emissions treatment equipment 120 (shown in FIG. 1) is coupled among heat exchangers 202, 204, and 206, evaporators 208, 210, and 212, and duct burners 220 to facilitate reducing an amount of contaminants entrained within the flow of exhaust gases. In an alternative embodiment, emissions treatment equipment 120 is positioned in the flow of exhaust gases downstream from heat exchangers 118.

FIG. 3 is a simplified block diagram and an exemplary emissions profile for HRSG 116. As shown in FIG. 3, HRSG 116 includes a first duct burner 302 that elevates a temperature of the exhaust gases after the exhaust gases have been discharged from gas turbine engine 106 via exhaust duct 114 (shown in FIG. 1). High pressure evaporator 208 is coupled in flow communication downstream from first duct burner 302, and also raises the temperature of the exhaust gases. For example, in one embodiment, first duct burner 302 and high pressure evaporator 208 receive the exhaust gases at a temperature of approximately 700° Fahrenheit (° F.). In the example, the exhaust gases include approximately 90 parts per million (ppm) of NO_(x), which includes approximately 10% nitrogen dioxide (NO₂), and first duct burner 302 and high pressure evaporator 208 heat the exhaust gases to a temperature above approximately 700° F. Moreover, in the exemplary embodiment, a high pressure heat exchanger 304 is coupled in flow communication downstream from high pressure evaporator 208 to facilitate cooling the exhaust gases to approximately 400° F.

In the exemplary embodiment, a second duct burner 306 is coupled in flow communication downstream from high pressure heat exchanger 304 to selectively increase the temperature of the exhaust gases to facilitate reducing the concentration of, for example NO_(x), in emissions channeled to stack 122. More specifically, in one embodiment, second duct burner 306 heats the exhaust gases to a temperature of between approximately 500° F. and approximately 800° F. In the exemplary embodiment, a carbon monoxide (CO) catalyst 308 is coupled in flow communication downstream from second duct burner 306 to facilitate oxidizing the nitric oxide (NO) to equilibrium concentrations of NO₂ at local exhaust temperatures at the location of CO catalyst 308 in the exhaust gas stream. Specifically, CO catalyst 308 oxidizes NO such that the exhaust gases include approximately 90 ppm NO_(x), which includes less than approximately 50% NO₂, but is SCR catalyst dependent.

In the exemplary embodiment, an injection apparatus 310 is coupled in flow communication downstream from CO catalyst 308. Injection apparatus 310 injects a reducing agent into the flow of exhaust gases to facilitate reducing the concentration of NO_(x) within the exhaust gases. Moreover, in the exemplary embodiment, a NO_(x) reduction element 312 is coupled in flow communication downstream from injection apparatus 310. NO_(x) reduction element 312 channels the exhaust gases, including the reducing agent, over a suitable reduction catalyst to facilitate reducing the concentration of NO_(x). For example, the reduction catalyst provides an environment suitable for the reduction of NO_(x), by 120 ppm of NH₃, to predominantly NO₂, such as approximately 80% NO₂. For example, in the exemplary embodiment, NO_(x) reduction element 312 channels the exhaust gases over the reduction catalyst at a temperature between approximately 500° F. and 800° F., wherein NO_(x) reduction element 312 reduces the NO_(x) to a concentration of approximately 9 ppm, which includes approximately 80% of NO_(x) as NO₂ and 5 ppm NH₃.

In one embodiment, injection apparatus 310 is an ammonia (NH₃) injection grid that is coupled in flow communication downstream from CO catalyst 308. NH₃ injection grid 310 injects ammonia into the flow of exhaust gases to facilitate reducing the concentration of NO_(x) within the exhaust gases. Specifically, NH₃ injection grid 310 injects a gaseous ammonia mixture into the flow of exhaust gases such that a concentration of ammonia is reduced to approximately 120 ppm. In an alternative embodiment, injection apparatus 310 injects a reducing agent such as hydrogen, or an organic reducing agent, such as a hydrocarbon material, into the flow of exhaust gases.

Moreover, in the exemplary embodiment, a NO_(x) reduction element 312, such as a selective catalytic reduction (SCR) element, is coupled in flow communication downstream from NH₃ injection grid 310. NO_(x) reduction element 312 channels the exhaust gases over a suitable reduction catalyst at a temperature between approximately 500° F. and 800° F. to facilitate reducing the concentration of NO_(x). In one embodiment, NO_(x) reduction element 312 uses an organic reducing agent, such as a hydrocarbon material, to reduce the concentration of NO_(x). For example, in one embodiment, NO_(x) reduction element 312 includes one or more catalysts provided in one or more catalyst zones. The exhaust gases, including the organic reducing agent, are routed through each catalyst zone to interact with the corresponding catalyst. Exemplary catalysts include, but are not limited to only including, a zeolite material, a catalytic metal such as platinum group metals, gallium, and/or a promoting metal such as silver, gold, vanadium, zinc, titanium, tin, bismuth, cobalt, molybdenum, tungsten, indium, and mixtures thereof. In an alternative embodiment, NO_(x) reduction element 312 uses a hydrogen-based reducing agent, such as H₂. Typically, the precious metal catalysts used in NO_(x) reduction element 312 having H₂ or hydrocarbon as a reducing agent require lower temperatures for the same NO_(x) removal efficiency as NO_(x) reduction element 312 having ammonia as the reducing agent. However, use of H₂ or a hydrocarbon reducing agent enables a lower maximum temperature before degrading or oxidizing ammonia to NO_(x) or both. Accordingly, in some embodiments, the precious metal catalysts are located closer to the exhaust of HRSG 116 in a lower temperature operating environment.

Furthermore, in the exemplary embodiment, HRSG 116 includes a low pressure heat exchanger 314 coupled in flow communication downstream from NO_(x) reduction element 312. Moreover, in the exemplary embodiment, low pressure heat exchanger 314 cools the exhaust gases to a temperature of approximately 150° F. including approximately 9 ppm NO_(x) and 5 ppm NH₃.

In the exemplary embodiment, a controller 316 is coupled to, for example, HRSG 116. Controller 316 controls activation and/or performance of second duct burner 306. For example, during a startup of combined cycle power plant 100 (shown in FIG. 1), controller 316 activates second duct burner 306 to heat the flow of exhaust gases to facilitate reducing NOx to, for example, NO and NO2, prior to the exhaust gases being released into the atmosphere via stack 122. The startup of plant 100 may be any of a hot start, a warm start, or a cold start. Duct burner 306 can operate during the purge stage of gas turbine startup. In some embodiments, controller 316 causes second duct burner 306 to heat the flow of exhaust gases during any transient operational phase or load or low speed, to facilitate an increased reduction of NOx by heating the exhaust gases to a temperature sufficient for NOx reduction to the desired emission level.

In some embodiments, the term “controller” refers generally to any programmable system including computers, systems, microcontrollers, reduced instruction set circuits (RISC), application specific integrated circuits (ASIC), programmable logic circuits (PLC), and any other circuit or processor capable of executing the functions described herein. The above examples are exemplary only, and thus are not intended to limit in any way the definition and/or meaning of the term controller.

Although the present invention is described in connection with an exemplary power generation system environment, embodiments of the invention are operational with numerous other general purpose or special purpose power generation system environments or configurations. The power generation system environment is not intended to suggest any limitation as to the scope of use or functionality of any aspect of the invention. Moreover, the power generation system environment should not be interpreted as having any dependency or requirement relating to any one or combination of components illustrated in the exemplary operating environment.

Table 1, below, provides exemplary, typical, non-limiting temperature and emissions concentration of the flow of exhaust gases from gas turbine engine 106 at locations within the exhaust stream upstream 318 from first duct burner 302, downstream 320 from high pressure heat exchanger 304, downstream 322 from high pressure evaporator 208, downstream 324 from CO catalyst 308, downstream 326 from injection apparatus 310, downstream 328 from NO_(x) reduction element 312, and downstream 330 from low pressure heat exchanger 314.

TABLE 1 TEMPERATURE AND EMISSIONS CONTENT OF EXHAUST GASES IN HRSG Location 318 320 322 324 326 328 330 Temp. (° F.) 700 700 400 500-800 500-800 500-800 150 NO_(x) ppm 90 90 90 90 90 9 9 NO₂ % 10 10 10 80 80 80 >80 NH₃ ppm 0 0 0 0 120 5 5

FIG. 4 is a side view of an exemplary commercial duct burner by available from Coen Company, Inc., of Foster City, Calif., USA, which may be used as second duct burner 306. However, it should be understood that any suitable duct burner may be used as second duct burner 306, such as a V-gutter coupled to a fuel pipe. In the exemplary embodiment, second duct burner 306 facilitates low speed and/or low load operation during transient operation of gas turbine engine 106 (shown in FIG. 1) and/or HRSG 116 (shown in FIGS. 1-3). Moreover, in the exemplary embodiment, second duct burner 306 includes a substantially circular housing 402 that defines a fuel inlet 404 that is coupled to a fuel source (not shown). Second duct burner 306 also includes a main body 406 that is coupled to housing 402. In the exemplary embodiment, main body 406 includes an upper portion 408 and a lower portion 410. Upper portion 408 includes an upper flange 412 that extends upward from housing 402 to form a guide that facilitates channeling the flow of exhaust gases over second duct burner 306. Moreover, upper portion 408 includes a plurality of fuel circuits 414 that are coupled in flow communication with fuel inlet 404. Similarly, lower portion 410 includes a lower flange 416 that extends downward from housing 402 to facilitate channeling the flow of exhaust gases under second duct burner 306. Moreover, lower portion 410 includes a plurality of fuel circuits 414 that are coupled in flow communication with fuel inlet 404. Upper flange 412 and lower flange 416 are each positioned to enhance the flow of exhaust gases over and under second duct burner 306 at a desired velocity that facilitates the flow of exhaust gases being exposed to flames emitted by second duct burner 306 for a desired amount of time. In the exemplary embodiment, second duct burner 306 also includes a plurality of compressed air outlets 418 that are positioned between upper portion fuel circuits 414 and lower portion fuel circuits 414. Fuel circuits 414 and compressed air outlets 418 are oriented to create a mixture optimized for combustion of the fuel emitted by fuel circuits 414 for use in heating the flow of exhaust gases.

FIG. 5 is a graph 500 that shows an emissions curve 502 of NO_(x) emissions over time from a combined cycle power plant, such as power plant 100 (shown in FIG. 1), without using second duct burner 306 to heat the exhaust gases before they are channeled to NO_(x) reduction element 312 (both shown in FIG. 3). Graph 500 also shows a reduced emissions curve 504 of NO_(x) emissions over time from power plant 100 when using second duct burner 306 as described above. As shown in FIG. 5, emissions curve 502 indicates that a high output of NO_(x) until the exhaust gases are at a temperature sufficiently high that enables NO_(x) reduction element 312 to effectively reduce the amount of NO_(x) in the exhaust gases, at portion 506. Moreover, transient start up and shut down emissions, shown by reduced emissions curve 504, are reduced by operating NO_(x) reduction element 312 at the required exhaust temperature at all times. Likewise to increase emission compliant operation at lower loads (lower energy output), as shown by portion 508 of emissions curve 502. Emissions are becoming regulated as a total number of tons per year at any time or engine condition. In addition, time 510 indicates a potential to start second duct burner 306 before gas turbine engine 106 (shown in FIG. 1) is ignited to further reduce start up emissions. Similarly, second duct burner 306 may be used during shut down to maintain the required SCR inlet temperature to minimize NO_(x) emissions.

FIG. 6 is a graph 600 that shows an amount of NO_(x) destruction (as a percentage) 602 based on an inlet temperature of NO_(x) reduction element 312 (shown in FIG. 3). The ability of NO_(x) reduction element 312 to destroy NO_(x) increases with the exhaust temperature. More specifically, the rate of reaction increases with the exhaust temperature. At low temperature the NO_(x) destruction is small and a large amount of ammonia is released from stack 122 (shown in FIG. 1). Second duct burner 306 (shown in FIG. 3) is regulated and/or controlled to heat the exhaust gases to the desired exhaust temperature to achieve the NO_(x) removal required. As the exhaust heats up, second duct burner 306 is turned down. As such, a first portion 604 of curve 602 indicates that, if the exhaust temperature is too low, a reaction rate within NO_(x) reduction element 312 is too low to effectively reduce the amount of ammonia released from stack 122. A second portion 606 of curve 602 indicates that, at within the desired temperature range described above, the temperature of the exhaust gases enables a sufficiently high reaction rate within NO_(x) reduction element 312 to reduce NO_(x) amounts as described above. Furthermore, a third portion 608 of curve 602 indicates that, the exhaust temperature is allowed to be too high, the reaction rate within NO_(x) reduction element 312 increases for NH3 oxidation to NO_(x) reducing effect NO_(x) removal rate increasing the NO_(x) released from stack 122.

Exemplary embodiments of methods and apparatus for use in reducing emissions, such as NO_(x) emissions, are described herein. The embodiments described herein facilitate increasing a temperature of exhaust gases from a gas turbine engine by using a duct burner to enhance an ability of a selective catalytic reduction (SCR) element to reduce the concentration of NO_(x) of the exhaust gases emitted into the atmosphere via an exhaust stack.

Exemplary embodiments of systems, apparatus, and methods of assembly are described above in detail. The systems, apparatus, and methods of assembly are not limited to the specific embodiments described herein but, rather, operations of the methods and/or components of the system and/or apparatus may be utilized independently and separately from other operations and/or components described herein. Further, the described operations and/or components may also be defined in, or used in combination with, other systems, methods, and/or apparatus, and are not limited to practice with only the systems, methods, and storage media as described herein.

A controller, such as those described herein, includes at least one processor or processing unit and a system memory. The controller typically has at least some form of computer readable media. By way of example and not limitation, computer readable media include computer storage media and communication media. Computer storage media include volatile and nonvolatile, removable and non-removable media implemented in any method or technology for storage of information such as computer readable instructions, data structures, program modules, or other data. Communication media typically embody computer readable instructions, data structures, program modules, or other data in a modulated data signal such as a carrier wave or other transport mechanism and include any information delivery media. Those skilled in the art are familiar with the modulated data signal, which has one or more of its characteristics set or changed in such a manner as to encode information in the signal. Combinations of any of the above are also included within the scope of computer readable media.

The order of execution or performance of the operations in the embodiments of the invention illustrated and described herein is not essential, unless otherwise specified. That is, the operations may be performed in any order, unless otherwise specified, and embodiments of the invention may include additional or fewer operations than those disclosed herein. For example, it is contemplated that executing or performing a particular operation before, contemporaneously with, or after another operation is within the scope of aspects of the invention.

When introducing elements of aspects of the invention or embodiments thereof, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements.

This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims. 

1. A method of providing a heat recovery steam generator (HRSG) for use with a gas turbine engine, said method comprising: coupling at least one duct burner within the HRSG; and coupling at least one oxides of nitrogen (NO_(x)) reduction element downstream from the at least one duct burner, wherein the at least one duct burner is configured to operate during gas turbine engine operations including startup, shutdown, and low-load operations with insufficient exhaust temperatures at the NO catalyst inlet for desired NO_(x) reduction to increase a temperature of exhaust gases routed through the at least one NO_(x) reduction element to enable the at least one NO_(x) reduction element to facilitate NO_(x) reduction reactions during the gas turbine engine operations.
 2. A method in accordance with claim 1, further comprising coupling an injection apparatus downstream from the at least one duct burner and upstream from the at least one NO_(x) reduction element, wherein the injection apparatus is configured to inject a reducing agent into the exhaust gases.
 3. A method in accordance with claim 1, further comprising coupling the at least one duct burner to a controller that is configured to selectively activate the at least one duct burner based on a temperature of the exhaust gases upstream from the at least one duct burner
 4. A method in accordance with claim 3, wherein coupling the at least one duct burner to a controller further comprises configuring the controller to activate the at least one duct burner to heat the exhaust gases during the gas turbine startup.
 5. A method in accordance with claim 1, wherein the at least one duct burner includes a plurality of fuel circuits, said method further comprising coupling the plurality of fuel circuits to at least one fuel source.
 6. A heat recovery steam generator (HRSG) coupled to a gas turbine engine that discharges a flow of exhaust gases including oxides of nitrogen (NO_(x)), said HRSG comprising: at least one duct burner for heating the exhaust gases; and at least one NO_(x) reduction element coupled downstream from said at least one duct burner and configured to facilitate reducing an amount of NO_(x) in the exhaust gases that are channeled into said at least one NO_(x) reduction element.
 7. An HRSG in accordance with claim 6, wherein said at least one duct burner is configured to be selectively activated based on a temperature of the exhaust gases upstream from said at least one duct burner.
 8. An HRSG in accordance with claim 6, wherein said at least one duct burner is configured to heat the exhaust gases to a temperature above approximately 500 degrees Fahrenheit.
 9. An HRSG in accordance with claim 6, wherein said at least one duct burner is configured to heat the exhaust gases during any of a startup phase, a shut down phase, and a load condition with insufficient exhaust temperatures at the NO_(x) catalyst inlet for desired NO_(x) reduction.
 10. An HRSG in accordance with claim 6, wherein said duct burner comprises a plurality of fuel circuits.
 11. An HRSG in accordance with claim 6, further comprising an injection apparatus coupled downstream from said at least one duct burner and upstream from said at least one NO_(x) reduction element, wherein said injection apparatus is configured to inject a reducing agent into the exhaust gases to facilitate NO_(x) reduction reactions.
 12. An HRSG in accordance with claim 6, wherein the flow of exhaust gases also includes carbon monoxide (CO), said HRSG further comprising a CO oxidation catalyst element coupled downstream from said at least one duct burner and upstream from said injection apparatus.
 13. An HRSG in accordance with claim 6, further comprising: a first heat exchanger coupled upstream from said at least one duct burner; and at least a second duct burner coupled upstream from said first heat exchanger.
 14. An HRSG in accordance with claim 13, further comprising a second heat exchanger coupled downstream from said at least one NO_(x) reduction element.
 15. A combined cycle power plant comprising: a gas turbine engine configured to combust a fuel in air to produce shaft power and a flow of exhaust gases including oxides of nitrogen (NO_(x)); and a heat recovery steam generator (HRSG) coupled in flow communication with said gas turbine engine, said HRSG comprising: at least one duct burner for heating the exhaust gases; and at least one NO_(x) reduction element coupled downstream from said at least one duct burner and configured to facilitate reducing an amount of NO_(x) in the exhaust gases channeled into said at least one NO_(x) reduction.
 16. A combined cycle power plant in accordance with claim 15, wherein said at least one duct burner is configured to be selectively activated based on a temperature of the exhaust gases upstream from said at least one duct burner.
 17. A combined cycle power plant in accordance with claim 15, wherein said at least one duct burner is configured to heat the exhaust gases to a temperature above approximately 500 degrees Fahrenheit or above a catalyst light off temperature.
 18. A combined cycle power plant in accordance with claim 15, wherein said at least one duct burner is configured to heat the exhaust gases during a transient operation phase of said combined cycle power plant.
 19. A combined cycle power plant in accordance with claim 15, wherein said at least one duct burner comprises a plurality of fuel circuits.
 20. A combined cycle power plant in accordance with claim 15, wherein said HRSG further comprises an injection apparatus coupled downstream from said at least one duct burner and upstream from said at least one NO_(x) reduction element, wherein said injection apparatus is configured to inject a reducing agent into the exhaust gases to facilitate NO_(x) reduction reactions. 